History of Related Art
In telecommunications systems, special subscriber features such as call waiting, call forwarding, abbreviated dialing, etc., are becoming increasingly important to both individual subscribers (for reasons of added convenience) as well as to telecommunications providers (because such features are sources of additional revenue). Such features are generally provided by special programming in the software of the central office exchange serving a particular subscriber. That is, the local exchange switch software is separately programmed to provide special service features to the subscribers connected thereto. Often both the hardware and the software of an exchange must be upgraded in order to enable the provision of special subscriber functionality. Calls between two parties connected to different exchanges are completed via a so-called transit or tandem exchange which forms part of the network interconnecting individual central office switches to one another. In those calls, the transit exchange is transparent to the two parties to the call and simply provides a voice path between the two end offices. Any special service features invoked by either party are provided by the end office to which that subscriber is connected, independent of the network connection between the two parties.
In most telecommunication systems providing plain old telephone service (POTS), the communications link between a calling party (A-party) and the called party (B-party), is under the control of the A-party. That is, the communication link remains in place until the A-party's telephone instrument is placed "on-hook" and the system breaks the communication links in the end offices of both parties and in any transit exchange(s) used to link the end offices together. Placing the called party's (B-party) telephone instrument on-hook has little effect until after a period on the order of several minutes (e.g., four minutes) after which a timer expires triggering the disconnection of the circuits between the calling and called parties. In certain other types of telecommunication services, such as integrated services digital network (ISDN), B-party disconnect is employed but the mechanisms for implementing it are considerably different from those of conventional POTS networks.
Providing special subscriber services within conventional telecommunication exchanges requires an extensive upgrading of the software of each and every individual exchange which is to furnish such special services to its customers. Such upgrading of exchanges is often extremely expensive and virtually prohibitive from a cost effectiveness standpoint with respect to the additional revenue provided by the added subscriber services. This is particularly prevalent in small towns or rural areas where the demand for special subscriber services is relatively low and where existing exchanges have been place for a considerable period of time and continue to adequately serve the basic telecommunications needs of the subscribers.
In exchanges which contain the enhanced software necessary to provide special subscriber services, the mechanism by which such subscriber services are triggered in the exchange is that of a subscriber instrument switch hook "flash", i.e., a brief depression of the instrument switch hook to signal the system that special services are required. For example, a short depression of the switch hook by one party is used to place the other party on hold and speak with a third party in response to a call waiting tone. A second switch hook flash restores the connection with the other party and disconnects the third party. Such hook flashes are terminated solely in the end office to which the subscriber sending the hook flash is connected. That is, the signalling function provided by the switch hook flash is only detected in the local central office to which the sending subscriber is connected and does not reach any transit exchange through which that office may be connected to another local central office. It is only when the time period the switch hook is depressed exceeds a first extended period of time that the local switch interprets the flash as a calling party disconnect signal (rather that a special services invocation signal) to then interrupt the connection. A switch hook flash of a second considerably longer period of time is necessary in order to signal a called party disconnect. In either case, an actual switch hook flash is transparent to the transit exchange and it is only in response to disconnect signals received from the end offices that causes it to disconnect its interoffice network connection.
It would be highly desirable to have a means by which special subscriber services could be rendered at the transit exchange level so that one transit exchange could serve many different local exchanges and thereby minimize the cost of a service provider to offer special services to its subscribers. That is, if special services were provided at the tandem or transit exchange level, as opposed to the local end office level, the hardware/software modifications necessary in the transit exchange would be more cost effective than the upgrading of each and every separate end office as is currently the case.
Comparatively recently, the concept of a universal telephone number (UTN) has been promoted in which a subscriber would have a single telephone number which would identify that subscriber to a telecommunications network regardless of the individual network terminal at which that subscriber is located. The network would maintain a data base so that once the subscriber having the UTN notifies the network of the current access terminal at which the subscriber is located, by calling the network and entering that information, the network can locate the subscriber in the event any calls are placed to the UTN. This concept greatly enhances the mobility of a subscriber and in addition, allows that subscriber to be provided with special subscriber services associated with his UTN. The subscriber can then be provided with the desired complement of special services regardless of where that subscriber originates or receives a call.
UTN subscribers reasonably expect to be provided with their special services regardless of where they are physically located. This expectation is difficult to meet in the event that special subscriber services can be provided only by virtue of special software and programming contained within the end office to which the subscriber is connected. One barrier against providing special services within a transit exchange is that in conventional POTS telecommunication networks, transit exchanges are essentially transparent to the telecommunications link. That is, it is virtually impossible to use in-band DTMF signalling to control special subscriber services functionality since that would prevent the use of such signalling to control other modem telephone applications such as banking services.
Thus, it would be highly desirable to be able to provide some means for providing within the transit exchange a capability of implementing special service features. This would require means for signalling to the transit exchange that a party desires to invoke and control special subscriber service features.